Normal human heart function results from a complex interplay of a large number of the biological subsystems which comprise the human organism. Despite the considerable amount of information which has been accumulated over many years in the field of cardiology, a complete understanding of this interplay is still lacking. In its place, however, there exists a sophisticated body of knowledge which relates many of the deviations from nominal heart function to specific pathologies of one or more of these subsystems. The search for reliable diagnostic tools to detect aberrant cardiac behavior, because of its obvious relevance to human health, has always been at the forefront of medical research.
In the majority of situations, the prediction of serious cardiac malfunction proceeds by necessity in an indirect, statistical fashion by assigning risk factors associated with other known pathologies (diabetes and hypertension are two examples), or behavioral traits (such as smoking or heavy alcohol consumption), which medical experience relates to a possible eventual cardiac dysfunction. A different category of risk factors are those associated with a subject's prior history of heart disease or possible genetic predisposition to it. In those cases where none of the usual risk factors indicate the probable occurrence of future heart disease, the characterization of the state of health of the human heart using standard techniques is, at present quantitatively imprecise.
Non-invasive techniques, such as those based on the display of electric surface potentials by an electrocardiograph (ECG), have been developed and perfected over the years to provide a physician with an often clear view of many of the more serious existing cardiac function abnormalities. These methods, however, are not sufficiently powerful to be able to predict future cardiac dysfunction, except in the statistical sense described above, and then only in a relatively small fraction of cases.
Further information about the state of cardiac health of a subject can be obtained through use of other, invasive techniques. While often the invasive techniques provide more detailed information about some aspects of cardiac function, these involve surgical procedures which entail a measure of risk for the subject, and require a generally elaborate clinical setting and the intervention of a highly skilled specialist, generally resulting in very high costs. For these reasons, non-invasive techniques are generally preferred over invasive ones, and a need clearly exists for new non-invasive cardiac-analysis methods to increase the amount of relevant information which can be gathered and presented to the clinician for diagnosis.
In addition, clinical diagnosis using the standard methods of electrocardiography typically proceeds through the identification of the shape of various heartbeat amplitude waveforms, as well as through the identification of abnormally long or short duration of various characteristics of such waveforms. This type of analysis, with the ensuing identification of abnormal function, generally requires evaluation by an experienced physician who has an extensive knowledge of possible waveforms, and the analysis is therefore, to an extent, a subjective process. In some cases, automatic identification of some of the most common abnormalities which can be seen in the shape of these waveforms is possible by comparing the waveform corresponding to an average heartbeat with a database of stored waveforms, but in most cases a skilled physician is required to analyze such waveforms.
Thus, the outputs from ECG equipment have not heretofore been put in a form or analyzed in a way so as to permit reasonably reliable conclusions to be reached automatically or semiautomatically. Further, such analysis is only able to detect existing abnormalities rather than being able to predict the occurrence of serious cardiac dysfunction. For example, when heart measurements are being made on a patient during surgery or in critical care, an alarm sounds only when the patient is experiencing some type of crisis, rather than when the heart starts reacting in a manner which might be indicative of an oncoming crisis so that appropriate emergency procedures could be initiated to prevent the crisis situation from arising.
A need therefore exists for improved techniques for performing cardiac diagnosis and analysis which technique can be practiced non-invasively, for example by use of a standard ECG machine, while providing substantial information concerning the cardiac condition of the patient. Such procedures should preferably generate outputs which can be automatically recognized as indicative both of existing heart or heart related problems, preferably at an early stage in the onset of such problems before physical symptoms are apparent, and of cardiac conditions which are precursors of a future heart or heart related problem. To increase the usefulness of such information, and to reduce costs, such information should be provided in a form which may either be automatically analyzed by a computer or other equipment or which may be easily read and deciphered by relatively inexperienced medical personnel.